Conventionally, as a fluorescent light source device, there has been a known device with a configuration in which a fluorescent material is irradiated with laser light as excitation light, and the fluorescent material emits fluorescence.
As shown in FIG. 4, a certain type of such a fluorescent light source device includes a flat plate-shaped fluorescent plate 51 composed of a fluorescent material, and an excitation light source 11 for irradiating excitation light to the fluorescent plate 51 (see, for example, Patent Literature 1). The fluorescent plate 51 is disposed on a heat dissipation substrate 31 via a bonding portion 52 formed of an organic adhesive, an inorganic adhesive, a low-melting point glass, a metal braze, or the like. The heat dissipation substrate 31 has a function of dissipating heat from the fluorescent plate 51 to the outside, and also has a function of serving as a reflection surface and a function of holding the fluorescent plate 51. In the fluorescent plate 51, a surface opposite to the bonding surface with the heat dissipation substrate 31 serves as an excitation light incident surface as well as a fluorescence emission surface, a partial region of the excitation light incident surface (specifically, a central region thereof) serves as an excitation light irradiation region, and the excitation light irradiation region and a peripheral region thereof serve as a fluorescence emission region.
However, in such a reflection type fluorescent light source device, there is a problem that the fluorescent plate is heated by the irradiation of the excitation light to increase the temperature of the fluorescent plate, and as a result, temperature quenching occurs in the fluorescent material in the fluorescent plate, and so a sufficient fluorescent light flux (fluorescent light quantity) cannot be obtained.
The reason why the temperature of the fluorescent plate becomes high will be described. In the fluorescent plate, since the fluorescent material converts a part of the light energy into heat energy upon receipt of excitation light, heat is generated by irradiation with the excitation light. In the fluorescent plate, in particular, an adjacent portion immediately below the excitation light irradiation region on the excitation light incident surface, that is, a surface layer portion relating to the excitation light irradiation region on the excitation light incident surface side, becomes extremely high in temperature. However, since the heat dissipation substrate is disposed on the surface side opposite to the excitation light incident surface, heat generated in the fluorescent plate cannot be sufficiently discharged by the heat dissipation substrate, and therefore, temperature quenching occurs in the fluorescent plate.
Such a problem becomes conspicuous when the incident power of the excitation light (excitation energy of the excitation light) is large. That is, a sufficient fluorescent light flux cannot be obtained as compared to the incident power of the excitation light.
As another configuration of the fluorescent light source device, there is a configuration in which a flat plate-shaped fluorescent plate composed of a fluorescent material is bonded to a light-transmissive substrate with an adhesive or the like, one surface of the fluorescent plate, which is the bonding surface with the light-transmissive substrate, serves as an excitation light incident surface, and the other surface, which is a surface opposite to the bonding surface with the light-transmissive substrate, serves as a fluorescence emission surface. That is, in the fluorescent plate, one surface is irradiated with the excitation light through the light-transmissive substrate, and the fluorescence is emitted from the other surface. In such a transmission type fluorescent light source device, the light-transmissive substrate has thermal conductivity, and is connected to a cooling block made of metal.
However, even in such a fluorescent light source device, there is a problem that a sufficient fluorescent light flux cannot be obtained because the temperature of the fluorescent plate increases.